The present invention is related generally to radio frequency (RF) filters, and more particularly to a dielectric block band pass filter having improved capacitive inter-resonator coupling via metalization which produces a filter that is particularly well adapted for use in mobile and portable radio transmitting and receiving devices. This invention is related to the invention disclosed in U.S. Pat. No. 4,716,391.
Conventional dielectric filters offer advantages in physical and electrical performance which make them ideally suited for use in mobile and portable radio transceivers. Conventional multi-resonator filters include a plurality of resonators that are typically foreshortened short-circuited quarter-wavelength coaxial or helical transmission lines. The resonators are arranged in a conductive enclosure and may be inductively coupled one to another by apertures in their common walls. Other conventional filters may employ purely inductive coupling between resonators in a common dielectric block of material by preventing capacitive coupling between resonators. Capacitive coupling between dielectric block filter resonators has been employed in some types of filters (see U.S. patent application Ser. No. 656,121, "Single-Block Dual-Passband Ceramic Filter", filed in behalf of Kommrusch on Sept. 27, 1984). However, when a precise filter response is required, it has been found to be expensive to maintain the desired capacitive coupling for the filter response. Additionally, when a modification of filter parameters is needed, a complete redesign of the filter physical characteristics has traditionally been necessary.